Stomata of a plant open due to : -

According to the active $K^+$ transport theory (Levitt), in light $H^+$ ions are pumped out of the guard cells, and $K^+$ ions enter the guard cells from the surrounding subsidiary cells (influx of $K^+$).
$Cl^-$ ions and malate also accumulate, so the solute concentration of the guard cells rises and their water potential falls.
Water enters the guard cells by osmosis, they become turgid, and because of their thick inner walls and radially arranged microfibrils they bend apart, opening the stoma.
Efflux of $K^+$ causes closure (e.g. under the influence of ABA).
So stomata open due to influx of potassium ions.